1. Field of the Invention
The present invention generally relates to coating systems suitable for protecting components exposed to high-temperature environments, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a process of depositing a graded thermal/environmental barrier coating system on a composite substrate material.
2. Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys. While superalloys have found wide use for components throughout gas turbine engines, alternative materials have been proposed. For example, composite materials, particularly silicon-based composites with silicon carbide (SiC) as a matrix and/or as a reinforcing material, are currently being considered for high temperature applications, such as combustor liners, airfoils, nozzles and other hot section components of gas turbine engines.
In many high temperature applications, a protective coating is beneficial or required for a Si-containing material. For example, protection with a suitable thermal-insulating layer reduces the operating temperature and thermal gradient through the material. Additionally, such coatings should provide environmental protection by inhibiting the major mechanism for degradation of Si-containing materials in a corrosive water-containing environment, namely, the formation of volatile silicon monoxide (SiO) and silicon hydroxide (Si(OH)4) products. Consequently, besides low thermal conductivity, a critical requirement of a thermal barrier coating system for a Si-containing material is stability in high temperature environments containing water vapors. Other important properties for the coating material include a coefficient of thermal expansion (CTE) compatible with the SiC-containing material, low permeability for oxidants, and chemical compatibility with the Si-containing material and silica scale formed from oxidation. As a result, suitable protective coatings for turbine engine components formed of Si-containing materials essentially have a dual function, serving as a thermal barrier and simultaneously providing environmental protection. A coating system having this dual function may be termed a thermal/environmental barrier coating (T/EBC) system.
Various single-layer and multilayer T/EBC systems have been investigated for use on Si-containing substrates. Coatings of zirconia partially or fully stabilized with yttria (YSZ) as a thermal barrier layer exhibit excellent environmental resistance. However, YSZ does not adhere well to Si-containing materials (SiC or silicon) because of a CTE mismatch (about 10 ppm/xc2x0 C. for YSZ as compared to about 4.9 ppm/xc2x0 C. for SiC/SiC composites). Mullite (3Al2O3.2SiO2) has been proposed as a bond coat for YSZ on Si-containing substrate materials to compensate for this difference in CTE (mullite having a CTE of about 5.5 ppm/xc2x0 C.), though mullite exhibits significant silica activity and volatilization at high-temperatures if water (water vapor) is present. Barium-strontium-aluminosilicate (BSAS) coatings have also been proposed as a protective coating for Si-containing materials in view of its excellent environmental protection properties and low thermal conductivity. In addition, BSAS has been proposed as a bond coat for YSZ in U.S. Pat. No. 5,985,970 to Spitsberg et al., assigned to the assignee of the present invention.
As application temperatures increase further beyond the thermal capability of a Si-containing material (limited by a melting temperature of about 2560xc2x0 F. (about 1404xc2x0 C.) for silicon), relatively thick coatings capable of withstanding higher thermal gradients are required. However, as coating thicknesses increase, strain energy due to the CTE mismatch between individual coating layers and the substrate increases as well, which can cause debonding and spallation of the coating system. As a solution, U.S. Pat. No. 6,444,335 to Wang et al. discloses a compositionally-graded T/EBC system that exhibits improved mechanical integrity for high application temperatures. The T/EBC system includes an intermediate layer containing YSZ and mullite, alumina and/or an alkaline-earth metal aluminosilicate (preferably BSAS). The intermediate layer is used in combination with a mullite-containing layer that overlies the surface of a Si-containing substrate, a layer of an alkaline-earth metal aluminosilicate (again, preferably BSAS) between the mullite-containing layer and the intermediate layer, and a thermal-insulating topcoat of YSZ overlying the intermediate layer. An optional silicon bond layer may be deposited on the substrate prior to depositing the mullite-containing layer. The mullite-containing layer has a CTE above that of a Si-containing substrate but less than that of the YSZ topcoat, and therefore compensates for the difference in CTE between the Si-containing substrate and the other coating layers. In addition, the mullite-containing layer serves as a chemical barrier between BSAS layer and the Si-containing substrate to prevent interaction of BSAS with the silicon oxidation product (SiO2) at high temperatures. The BSAS layer provides environmental protection to the silicon-containing substrate, while the YSZ topcoat offers thermal protection to the Si-containing substrate and the other underlying layers of the coating system. Finally, the intermediate layer serves as a thermal barrier layer that also provides a CTE transition between the BSAS layer and the YSZ topcoat as a result of its BSAS, mullite and/or alumina content, each of which has a CTE between that of YSZ and Si-containing materials.
In view of the above, the compositionally-graded T/EBC disclosed by Wang et al. is able to reliably provide both thermal and environmental protection to a Si-containing substrate at high temperatures. Another desirable feature of the T/EBC of Wang et al. is that each of its ceramic layers can be readily deposited by known deposition techniques, particularly air plasma spraying. Nonetheless, further improvements are continuously sought. For example, though the coating system taught by Wang et al. makes use of a carefully tailored combination of coating materials, horizontal cracking and spallation has been observed in the intermediate layer and the YSZ topcoat following thermal cycling at high temperatures. Accordingly, it would be desirable if the microstructure and mechanical integrity of this coating system could be enhanced.
The present invention generally provides a process for depositing a ceramic coating system for Si-containing materials, particularly those for articles exposed to high temperatures, including the hostile thermal environment of a gas turbine engine. Examples of such materials include those with a dispersion of silicon carbide, silicon carbide and/or silicon reinforcement material in a metallic or nonmetallic matrix, as well as those having a silicon carbide, silicon nitride and/or silicon-containing matrix, and particularly composite materials that employ silicon carbide, silicon nitride and/or silicon as both the reinforcement and matrix materials (e.g., SiC/SiC ceramic matrix composites (CMC)).
The invention is particularly applicable, though not limited, to depositing the compositionally-graded T/EBC system disclosed by Wang et al., and is tailored to improve the mechanical integrity of this T/EBC system when deposited on silicon-containing substrates used in high temperature combustion environments. As such, coatings deposited with this invention comprise multiple ceramic layers with differing compositions, and particularly a dense, strain-tolerant, vertically-cracked YSZ-containing ceramic layer that is deposited on a second ceramic layer having a composition different than the YSZ-containing ceramic layer. Particularly suitable compositions for the second ceramic layer include the intermediate layer disclosed by Wang et al., namely, a layer containing YSZ and mullite, alumina and/or an alkaline-earth metal aluminosilicate (preferably BSAS), and especially a mixture consisting essentially of YSZ and either mullite or BSAS. The method entails depositing the YSZ-containing ceramic layer using a plasma spraying technique while maintaining the silicon-containing substrate at a temperature of not greater than about 600xc2x0 C., and more preferably not higher than about 450xc2x0 C. to about 550xc2x0 C., depending on the composition of the second ceramic layer on which the YSZ-containing ceramic layer is deposited.
According to this invention, a compositionally-graded T/EBC deposited on a silicon-containing substrate and comprising a dense, strain-tolerant, vertically-cracked YSZ-containing layer deposited on a second ceramic layer containing a mixture of YSZ and BSAS or mullite has been shown to exhibit improved mechanical integrity and thermal cycle fatigue life if the deposition process for the YSZ-containing layer is carefully controlled so that the temperature of the silicon-containing substrate does not exceed about 600xc2x0 C. A particularly suitable temperature for the substrate is about 450xc2x0 C. if the second ceramic layer (on which the YSZ-containing layer is deposited) contains YSZ and BSAS, and a particularly suitable temperature for the substrate is about 550xc2x0 C. if the second ceramic layer contains YSZ and mullite. Limiting the deposition temperature in this manner has been associated with the avoidance of fine horizontal cracks, which are believed to promote wrinkling and eventually spallation of the T/EBC, while maintaining the desired dense vertically-cracked structure of the YSZ-containing ceramic layer for strain tolerance. Therefore, by limiting the deposition temperature, the microstructure and mechanical integrity of the coating system can be enhanced, leading to a longer component life.
Other objects and advantages of this invention will be better appreciated from the following detailed description.